A video signal to be displayed on a display screen may either be analog data from a television camera, or digital data that is stored as a virtual screen in a display memory. To produce a color picture on the display screen, the signal controls a triple set of electron beams that scan across the surface of the screen from left to right and down the screen from top to bottom. When a beam reaches the end of a horizontal line, it is turned off and moved to the left edge of the screen one line down and begins to scan the next line. A picture created using this technique is called a raster scan.
The video signal includes a series of chromanance and luminance values that control the intensity of the electron beam, which in turn controls the amount of illumination produced at each pixel location in the scan line. As the electron beams pass over the phosphors that comprise each pixel the phosphors are exited and emit the desired color of light. After the beams pass, the light intensity begins to decay.
The rate of decay is called the phosphor's persistence. Long persistence means that it takes longer for the light intensity to decay and a short persistence means the light intensity decays quickly. The screen refresh rate can be slow when a long-persistence phosphor is used, but if the phosphor decays too slowly, moving images on the screen may appear to smear as they are moving.
When a short-persistence phosphor is used, a higher screen refresh rate must be used to refresh the decaying phosphors. The problem with high refresh rates is that they place high performance requirements on the display system. Early graphic systems refreshed the screen at a rate of 25 to 30 Hz, or 25 to 30 times a second. Even with long-persistence phosphors, it has been shown that this rate will produce a sensation of flicker, rather than a stable image.
Interlacing is a process that has long been used to reduce the amount of flicker on a display screen. During interlacing, an image is produced on the screen by scanning all the odd lines on the screen first, followed by the even lines. Although interlacing makes it difficult to detect flicker for large and fast moving objects, displaying objects that have high-contrast horizontal lines still results in annoying flicker.
A traditional method for reducing flicker even further is by passing the video signal through a flicker filter. Flicker filters attempt to reduce flicker by eliminating the high vertical frequency component of the video signal. Since flicker is caused by high contrast, eliminating the high frequency component of the video signal effectively smooths the signal, but also smears the signal across more scan lines. The problem with smearing the signal is that it reduces the resolution of the image. Therefore, traditional flicker filters force a trade-off between the amount of flicker the signal will produce and the signal's resolution. Traditional flicker filters also are indiscriminate in that they process color portions of the signals, rather than only the black and white flicker causing portions of the signal. Thus traditional flicker filters are inefficient and over process the data.
Accordingly, what is needed is an improved system and method for reducing flicker on a display screen. The present invention addresses such a need.